Question

Not sure what to search for here, so apologies if I’m repeating another question.

I’m wondering if there are any issues that I’m not aware of with using the following syntax in JavaScript:

var a = {};

var b = a.niceCoat = {};

Seems handy, but I just want to make sure…

Answer 1

That is perfectly fine, because a was declared previously. The expressions will be evaluated as

var a = {};
var b = (a.niceCoat = {});

I.e. it first assigns a new empty object to a.niceCoat and the result (the result of an assignment is the assigned value) to b.

But be aware of something like

var a = b = 'c';

which, again, is evaluated as

var a = (b = 'c');

Only a will be in local scope, b would be global. If you want b to be local too, you have to declare it beforehand: var b;. Something like var a = var b = .... does not work (not valid syntax).

---

Slightly off topic:

This method is indeed handy. Imaging you have an object of objects, something like:

var map = {
    foo: {},
    bar: {}
};

and you want to get the object for a certain key or create a new one if the key does not exists. Normally one would probably do:

var obj = map[key];
if(!obj) { // works because if the key is set, it is an object 
    obj = {}; //                                        which evals to true
    map[key] = obj;
}
// now work with obj

With the above method, this can be shortened to

var obj = map[key];
if(!obj) {
    map[key] = obj = {};
}

And we can make it even shorter with the logical OR operator (||):

var obj = map[key] || (map[key] = {});

(though it might be less readable).